text:
Can noise from contra-rotating propellers be reduced by having a different number of blades on each propeller?
Heads up: This came from Wikipedia's page on contra-rotating propellers and it explicitly says citation needed:

However they can be very noisy ... It is also helpful if the two propellers have a different number of blades (e.g. four blades on the forward propeller and five on the aft).[citation needed]

Any truth to this? Can a variable number of blades per propeller really help reduce noise? If so, how?
(It doesn't explicitly say if it's piston prop or turbo prop. AFAIK, both can be used for contra-rotating props, and I have no idea which one would be more noisy, or if this supposed variable-blade idea will work on one but not the other.)

I always thought this was a power recovery strategy, not a noise reduction one. Paging @PeterKämpf?

It does help a little, yes.
One source of noise is the fluctuation in dynamic pressure experienced by a blade of the rear propeller when it cuts through the wake of a blade of the forward propeller. Friction in the boundary layer on the first blade will reduce the dynamic pressure locally and create pressure fluctuations on the second blade, and noise.
By using a different number of blades, the pressure fluctuations will not happen all at the same time, but sequentially, which reduces the peak power of the noise. The effect of the second blade cutting through the wake of the first is still there, but is distributed over time.
Thanks to @MSalters for pointing out another effect. I copy straight from his comment:
The effect is not just distributed over time, but also shifted up in frequency. At 6000 RPM (100 Hz), with 4 blades per propeller they pass each other 8 times per revolution (800 Hz). If the second propeller has 5 blades, this moves up to 4000 Hz.

P-27 prop on the D-27 turboshaft of the An-70 transport, picture by Marina Lystseva. The forward propeller has 8 blades and the rear propeller has 6. For optimum noise reduction, the rear propeller should have just one blade less than the forward propeller. The leading edge sweep helps to stretch out the contact of a single blade with the wake over time, which again reduces peak noise.

The effect is not just distributed over time, but also shifted up in frequency. At 6000 RPM (100 Hz), with 4 blades per propeller they pass each other 8 times per revolution (800 Hz). If the second propeller has 5 blades, this moves up to 4000 Hz.

@MSalters: Right! I forgot, will edit the answer.

This is fascinating but for optimum reduction, I don't see why the rear prop should have 1 less blade than the front. Why not the front having 1 less than the rear? Why not any combination of different prime numbers?

@DrZ214: The rear blades work in a flow swirled by the forward propeller which gives them a better efficiency and higher loading when operated at the same incidence as the forward blades. If you want to neutralize torque, it is better to reduce the blade count on the rear prop.

Logic would suggest you want the number of blades on each propeller to be coprime in order to shift the frequency maximally. If you also want the number of blades to be similar, using a difference of 1 blade is probably the optimal solution.

Yes!
For c-r fan's tonal noise (one kind of noise and another one is broadband), it can be reduced a lot by set the modal mismatch index to be 2. For instance, if the no. of blade of one rotor is 7, the other one should be 9 or 5 for least radiated noise.

Neat. But what is tonal noise and what is broadband noise? What is the difference between them?


text:
What is the definition of taxiing?
According to Wikipedia the definition of taxiing is:

the movement of the aircraft on the ground under its own power [however]
  push-back is not considered taxiing

In contrast, Eurocontrol seems to define it as:

any movement on the ground including push-back

Any other reliable sources to look up for the correct definition?

I removed the request for an opinion, as that would be off-topic here.

@tcfx44 The link for EUROCONTROL's website you referenced does not provide the definition you mentioned.

Concerning the first definition: If neither movement under the aircraft's own power nor pushback count as taxiing, there isn't much left, or is there?

@O.R.Mapper I think you may have misread the first one, due to its somewhat awkward wording. He was saying that taxiing consists of moving on the ground under its own power and, therefore, push-back (which usually doesn't take place under the aircraft's own power) isn't considered taxiing.

Actually, your question will coming back to you. It depends on why do you ask the question you partly know the answer. If you are a pilot, then Wikipedia answer is enough. But if you are trying to write something on aviation, you probably should consider who is your target reader, and give them the most satisfiable definition.

@O.R.Mapper You had to read the quote and its introduction as a single sentence ("taxiing is foo and bar is not taxiing") rather than reading the quote on its own ("foo and bar is not taxiing"). I edited the quote to avoid this confusion.

Definitions always depend on their context, that is why documents often included a list of definitions. It allows authors to define terms in the context of the document. A definition context is not always a document, it could for example be a field of expertise as well.
The Commercial Aviation Safety Team / International Civil Aviation Organization (CAST/ICAO) Common Taxonomy Team (CICTT) taxi is defined as:

The aircraft is moving on the aerodrome surface under its own power
  prior to takeoff or  after landing

More detail can be found in the phase of flight definition document.
This is definition can be applied in many situations.
The Eurocontrol definition you link to comes from the Air Traffic Flow & Capacity  Management (ATFCM) and Airport Collaborative Decision Making (A-CDM) domains where statistics about gate-to-runway time play an important role for planning purposes.
In that context it is not really interesting to distinguish the time that was spend in pushback from the time spend actually taxiing to the runway. The taxi-out time is hence defined as the time between off-blocks and take-off.

The FAA definition is in 14 CFR 135.100 and the pilot/controller glossary (quoted here):

TAXI- The movement of an airplane under its own power on the surface
of an airport (14 CFR Section 135.100 [Note]). Also, it describes the
surface movement of helicopters equipped with wheels.

Interestingly, "pushback" is not in the glossary at all.
This matches the Wikipedia definition that you mentioned, although not the ICAO one from DeltaLima. I think this definition makes sense, because taxiing doesn't necessarily have to be before a takeoff or after a landing. It's possible that the aircraft is simply taxiing from one point to another on the airport surface, e.g. from the fuel pumps to the hangar. That might be more common in GA, though: when I see airliners repositioning on the ground they're usually being towed.
You also didn't mention air or hover taxiing, but I think it's fairly obvious in that case that the helicopter is under its own power. Here are the FAA definitions, for completeness, both from the P/CG:

AIR TAXI- Used to describe a helicopter/VTOL aircraft movement
conducted above the surface but normally not above 100 feet AGL. The
aircraft may proceed either via hover taxi or flight at speeds more
than 20 knots. The pilot is solely responsible for selecting a safe
airspeed/altitude for the operation being conducted.
HOVER TAXI- Used to describe a helicopter/VTOL aircraft movement
conducted above the surface and in ground effect at airspeeds less
than approximately 20 knots. The actual height may vary, and some
helicopters may require hover taxi above 25 feet AGL to reduce ground
effect turbulence or provide clearance for cargo slingloads.

"Helicopters equipped with wheels" -- really?

Yes, really: http://en.wikipedia.org/wiki/File:UH-60A_Black_Hawk.jpg

@BurhanKhalid Didn't you see Airwolf?

Air wolf! Of course.

@BurhanKhalid Really. Many helicopters are equipped with wheels rather than skids. (The Boeing Chinook was the first example that came to my mind; other examples include military helicopters designed for launch from ships: it's much easier to wheel the helicopter into the hangar than anything else.)

any movement on the ground including push-back

This definition of taxiing is more specific to controllers. Once the aircraft has commenced its pushback it is normally infringing a controlled movement area and will be under the control of the ground or tower controller.

the movement of the aircraft on the ground under its own power and
  push-back is not considered taxiing

I believe this to be the more correct definition. Movement of an aircraft under its own power is important in a regulatory sense for aircraft operators. There are a number of matching definitions, relating to compliance with the Minimum Equipment List(MEL), ETOPs despatch - relating to weather minima and the definition of a sector, which is important for calculating the allowable flight duty period(FDP).

Definition of ETOPs Dispatch:

Extended range Twin Operations

Dispatch
  Dispatch is when the aircraft first moves under its own power for the purpose of taking-off.

Dispatch is also used in relation to an aircraft meeting the requirements of the Minimum Equipment List (MEL) for a "flight"(defined below).

Definition of Sector
UK CAP371 Flight Time Limitations

'Sector'
  The time between an aircraft first moving under its own power until it next comes to rest after landing, on the designated parking position.

Flight

Definition of Flight - ref to MEL

"Commencement of flight" The point when an aircraft begins to move under its own power for the purpose of preparing for take off.

The time you leave the apron (GATE/RAMP position) and arrived at your takeoff strip (Piano key-marked Takeoff zone), the journey in between is well-known as taxiing. I am sure that this is correct because I have experience with runways and aircrafts. It's a common sense that taxii means no ground power/air cart, and also no support. You taxi your way to the takeoff zone. No disupte about this.

Please give a citation. The personal experience of an anonymous stranger on the internet isn't worth a lot because we have no way of evaluating whether your experience is typical or whether you're interpreting it correctly.

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sort: votes


text:
Do airplanes have any kind of fire insulation?
Commercial airplanes sometimes suffer from a bird strike or something else that could ignite a part of the engine. So apart from noise insulation, do airliners have fire insulation?

As well the APU comparment is protected agaisnt fire

Yes. As you could have found in seconds by searching for "aircraft fire insulation" with your favourite search engine.

Are you asking about the engines or the fuselage?

@TomMcW About anything that could serve as fire insulation

@DavidRicherby I could be wrong, but isn't the goal of the SE network to build a canonical set of questions and answers?  Something being searchable, even easily searchable, doesn't necessarily make it a bad question.

Yes, aircraft have fire-protection systems as well with many of the materials being of such a sort.
From boeing.com about the fire-resistant/fire-suppression systems on their aircraft. You can see that most of the aircraft is covered in these materials. Emphasis mine.

Interior components of Boeing airplanes meet flammability requirements prescribed in Title 14 Code of Federal Regulations (CFR) Part 25. These components include:
- Interior ceiling.
- Interior sidewall panels.
- Partitions.
- Galley surfaces and structure.
- Exposed surfaces of stowed galley carts and standard galley containers.
- Large cabinets and cabin stowage compartments.
- Passenger seat material.

For materials in areas not covered by the CFR requirements, Boeing design guidelines are used to identify additional flammability, smoke, and toxicity requirements.
  The standards for flammability of insulation blankets have improved over time. A recent requirement change calls for the enhancement of the fire-protective features of insulation blankets in the event of an in-flight or post-crash fire. The latest standard increases protection by minimizing the contribution of the insulation blankets to the propagation of a fire. Thermal/acoustic insulation installed behind cabin interior panels with the appropriate fire-resistant properties can delay the onset of fire into the cabin in the event of a crash (see fig. 1). The insulation blankets, along with the airplane skin, must be capable of resisting burn-through from a fuel-fed post-crash fire next to the bottom half of the fuselage for a minimum of four minutes to allow passengers to evacuate the airplane before burn-through can occur.

You can also find out much more in that webpage about the various active and passive fire-suppression systems in aircraft.
The FAA Regulation about these systems can be found here in PDF format.

authors: Андрей Махуков

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text:
Do gyroscopic forces (from rotating engines) significantly affect yaw or pitch?
It occurs to me that jet engines are very heavy objects operating at high RPMs. This must have a very high angular moment and thus a high gyroscopic force.
This would restrict rotation when trying to yaw or pitch. Rolling, however, would not be effected.
How big of a problem is this? I imagine it is a concern to military aircraft as they want to be as maneuverable as possible. But I'm very interested in commericial efficiency too. I wonder if the giant A380, for example, had to be given greater elevator and rudder authority to help it pitch up at takeoff.
Note: all engines rotate but I'm assuming the jet engine will have by far the highest angular moment due to the weight of their turbines and compressors. Turbofans with high bypass ratio probably have the greatest gyro forces due to their huge fan in addition to heavy spinning spools.
(Presumably, contra-rotating engines would have zero gyroscopic forces.)

I am pretty sure this is a duplicate and there is already great answer explaining it is not generally noticed in jets (it is noticed in props) except the Rolls-Royce Pegasus used in Hawker Siddeley Harriers, which has low and high pressure shafts rotating in opposite directions to reduce gyroscopic effects. The related question search being as bad as it is I can't find it now though.

Yes. The more so the higher the inertia and speed of the rotating masses and the lower the control authority. While the gyro forces of a jet are negligible in regular flight due to the high damping provided by the wings and tail, in hover they can become dominant. The Bristol-Siddeley Pegasus engine of the Kestrel, Harrier and AV-8B jump jets needs to have the low pressure spool run in opposite direction of the high pressure spool to balance its gyro effects. If that would not be the case, a yawing motion would produce a pitching motion, and vice versa. When you sit on a jet, the thrust of which is equivalent to your weight, tilting this jet fore or aft only slightly will produce a quick shift of your location, so any maneuvering in hover will become extremely hard.

USMC AV-8B Harrier in hover (picture source, © Richard Seaman 2001)
Generally, contra-rotation of engine parts has been used to reduce gyro effects. They first became an issue with rotary engines in WW I. A rotary engine has its crankshaft fixed to the airplane, and both the cylinder block and the propeller rotate. This gives better cooling at low speed and produces a flywheel effect, so the engine runs more smoothly. But when you yaw, the gyro effect pitches the aircraft up or down, so any precise maneuvering becomes very hard.
With the increasing engine power in 1916 and 1917, this effect became so severe that geared engines were developed where the cylinders rotate in one direction and the propeller in the opposite direction. As a consequence, the propeller had only half the RPM in air as it had with the cylinder block. This resulted in great propeller efficiency, but also big propeller diameters, so airplanes with those engines needed a high landing gear. Below is a picture of a Roland D XVI with a Siemens & Halske III contra-rotating rotary engine from 1918. This was an excellent fighter aircraft for its time with almost no gyro coupling.

Roland D XVI (picture source)
Generally, jet engines are sensitive to maneuvering: Changing load factors will bend the spools between their bearings such that the tip clearance of both the turbine and the compressor need to be big enough to avoid contact under g forces, or when rolling with a wing-mounted engine. Every engine manufacturer will publish a load factor envelope within which the engine is safe to operate. Widening this envelope will reduce engine efficiency because the gaps between the rotating parts and the engine housing need to grow with the maximum load factor.

The answer is yes.  Gyroscopic forces are a factor on propellor driven aircraft, but generally not noticeable on a stable ('God Fearing') aircraft in cruise flight.
However gyroscopic effects do become noticeable in several areas of a propellor driven aircraft's flight envelope.  One of the best known areas are in dramatic changes in pitch attitude at low speeds.  Tail wheel aircraft are particularly sensitive to this during the takeoff roll.  Fast transitions from a three point attitude to a wheel attitude during ground roll will cause noticeable and unwanted yaw inputs due to gyroscopic precession in addition to changes in P-Factor.  During takeoff in a typical tail wheel airplane with a propeller which rotates clockwise when viewed from the cockpit will yaw left when the tail is lifted during the takeoff roll and requires right rudder to counteract the gyroscopic precession.  Likewise during a wheel landing, precession from the propellor will cause the tail to yaw right as the the tail wheel settles onto the runway and requires left rudder to counteract.
Advanced or unlimited aerobatic aircraft often fly gyroscopic maneuvers such as the Lomcevak, which utilize gyroscopic precession to induce tumbling forces into maneuvers.  Even loops, hammerhead stalls, etc require a significant rudder or other control inputs to counter precession from the prop at slower speeds.

"God fearing aircraft" is now part of my training repertoire.  Love it.

Thanks, but just so you know, I'm interested if these things are significant for any engine type. Not just piston prop and turboprob, but turbojet and turbofan too.

@DrZ214: It's the props that cause most of the gyroscopic effect. The engines themselves are too small to matter. Even jet engines. In fact, one of the big advantages of jet engines touted by early jets is the lack of gyroscopic effects. (in theory, it doesn't lack it but the pilot can no longer notice it)

"from a three point attitude to a wheel attitude"? Is that statement missing a word?

@MichaelKjörling I assumed that "wheel attitude" meant that the tail was no longer dragging. I could be quite wrong.

One area not yet mentioned where gyroscopic effects are significant (in both jets and props) is spin. (In English it sounds almost tautological :) Spin always involves significant angular velocities - while having very limited control authority.
(In a way, this is similar to the effects on aerobatics mentioned by Carlo Felicione.)
Due to gyroscopic effects of the engine(s), most aircraft have different left and right spin characteristics. To the point that, say, left spin may be almost unrecoverable while right spin would be easily recoverable. This also applies to initiating the spin: the aircraft may tend to enter spin in one direction more willingly than in the other.

You do see gyroscopic effects when you look at WW1 fighters. Quote from the Wikipedia Article on the Sopwith Camel:

The type owed both its extreme manoeuvrability and its difficult handling to the close placement of the engine, pilot, guns and fuel tank (some 90% of the aircraft's weight) within the front seven feet of the aircraft, and to the strong gyroscopic effect of the rotating mass of the cylinders common to rotary engines.

For comparison, a 777-300ER has a max take-off weight of 775,000 pounds (reference), while one GE90-94B engine weighs 16,644 pounds, not
all of which would be rotating.  So if we assume half the weight is rotating mass (probably high, but I don't know), the rotational mass of both engines is only about 2% of the total mass.
Finally, I'm not aware of any contra-rotating jet engines, so that would imply the forces aren't too noticeable.

Yes, gas turbines can have noticeable gyroscopic effects at low airspeeds.
The Harriers RR Pegasus has contra-rotating spools for that reason.

This answer would be improved with referenced or citations to support it.

Not to be pedantic, but I need to be pedantic about this.  Many of the comments and answers on this post are talking about the effect of the engine's TORQUE on the aircraft, which is distinctly different from gyroscopic precession.  We fall too easily into the habit of calling any resulting artifact of a heavy, fast-turning object "gyroscopic effect" when gyroscopy is only one of several factors and usually not the most significant.

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text:
Can the safety of the Boeing 777 be extended to other models?
It is becoming accepted that the Boeing 777 is the world's safest aircraft. There must be a reason for this of course, and aircraft designers should know it.
So my question is, can't we take all the safety merits of a B777 and copy them into other models and bring all airplanes to the same level of safety?

retro fitting things into older airplanes is very expensive and not always possible

Why down vote? As an aviation enthusiast to me the question seems pertinent.

The safest airliner is the one that is built, towed into a hangar, and never flown, but you'll recall the old nautical adage: "A ship in harbor is safe, but that is not what ships are built for."

As soon as we figure out how to transfer luck, we really should do this!

What's the "safest aircraft" depends in large part on who operates it, how they operate it, and where.
For many operators the 777 will be their prestige airliner, being assigned the most senior, most experienced, crews, the best maintenance, etc. etc..
That alone will mean less incidents and accidents than other types. They also have the range and ceiling to be able to fly around or over most serious weather. A 30 minute detour on a 10 hour trip can be made up, a 30 minute detour on a 1 hour trip in a 737 or A320 can't be (even if the aircraft would have the spare fuel to fly that detour).
The rest is mostly statistics. I seriously doubt there have been statistically more accidents per hour flown or per rotation on say an A340 as compared to a 777. There have been more airframe losses, but those include several lost on the ground due to them being stuck on airports in warzones (think what happened on Sri Lanka where Tamil separatist mortar bombs destroyed several airliners on the ground, damaging the until then flawless safety record of the type).
There have been AFAIK 3 777 hull losses (including the missing MH370) in flight.
Given the far higher number of say 737s in operation, and the far longer service history of that type, often with operators with shady operational procedures (more than a few are for example blacklisted by the EU) it's little surprise there have been more hull losses among the type. But comparing say the 737-900 only with the 777, with somewhat similar technology and number in service, as well as similar prestige level for crew assignments, you'll find a similar safety record.

+1, I also think that that the 777 experiences a lower number of pressurisation cycles in the course of its lifetime than a shorter range 737. This makes it less susceptible to airframe stress and cracks than a 737.

@shortstheory both pressurization and takeoff/landing cycles have a lot to do with stress and wear. As 777s are basically exclusively long haul, they see much fewer of both.

@shortstheory true, but then the 737 also will encounter more frequent depot level maintenance. Maintenance during which there are checks performed for just those things IF procedures are followed correctly.

The 777 is also fairly new, at only 20 years old - that means they have fairly up to date safety features, and that they aren't being flown around far-flung, lightly regulated parts of the world by small cargo carriers and cowboy airlines.

@jwenting; a lot of things that get stressed by pressurization aren't part of a normal ground crew's checklist between flights, and that means they'd have to be obvious to be spotted. Those kinds of maintenance checks happen after a certain number of total hours in the air, regardless of whether the airframe spent those hours on X long-haul flights to Dubai or X * 10 business hops from Dallas to Houston.

Statistically, the A340 is actually safer than the 777 - not a single person has died in an A340, and only sixteen people have been seriously injured in one (AF358 [12]; Toulouse Blagnac ground test [4]), while 540 people have been killed on board 777s (OZ214 [3]; MH370 [239]; MH17 [298]), plus two ground personnel (BA2019 [1]; EK521 [1]), and 51 seriously injured (BA38 [1]; OZ214 [49]; EK521 [1]).

@Sean now, yes, after 2 hull losses in flight with all hands. In 2014, no.

@jwenting: Whoops, didn't notice the date on that answer... although the 777's safety record was already worse even in March 2014 (OZ214 crashed in July 2013).

The "safety" of an aircraft is a function of the entire aircraft, from concept to design to manufacturing and assembly, and also of the people who fly it.
Very old anecdote; in one of the very first "evaluations" performed on competing pre-production aircraft to determine the one an air service should buy, Manfred von Richthofen flew what would become the Fokker D.VII. He stated the plane was very unstable, especially in a dive. As a result, the fuselage was lengthened by one box section (about 2-3 feet). Only very minor modifications were made elsewhere. On flying the modified prototype, Richthofen said it was the best craft he'd ever flown, and on this recommendation the D.VII entered service and became so feared that the Armistice specifically ordered the surrender of all German D.VIIs to the Allied forces. So, just this one change to the airframe turned an underpowered, temperamental aircraft into the most successful fighter design of WWI.
Fast-forward 97 years or so, and not much has really changed regarding this basic tenet. What has changed is the inherent amount of complexity in the average aircraft. We think of something like a Piper Cub as being fairly simple, and compared to an F-15 or a 747, they are, but even relatively inexpensive modern hobbyist planes have avionics, radio systems etc that put the original J-3s to shame. More complexity means more that can go wrong, even when the complexity is specifically designed to lower pilot workload.


text:
Can two airports have a different ICAO code but share the same IATA code?
I am searching some airports databases and I find some airports with the same IATA code with different ICAO codes. Is it mistake in the database? Is it OK?
For example:

Beaufort MCAS - Merritt Field (ICAO KNBC IATA BFT)
Beaufort County Airport       (ICAO KARW IATA BFT)

Edit: Another example;

Paamiut Airport    (ICAO BGPT IATA JFR )
Paamiut Heliport   (ICAO BGFH IATA JFR )

Another example with 40 km between them:

Desierto de Atacama (IATA: CPO – ICAO: SCAT)
Chamonate           (IATA: CPO – ICAO: SCHA)

Definitely some confusion is going on somewhere. Note that Wikipedia sources its "BFT" for the MCAS to a database entry which is headed "Beaufort-County Airport", but shows Google Maps coordinates for the MCAS. Also, does the IATA usually assign any codes to military airbases with no civilian traffic?

IATA itself only lists BFT as the city code for Beaufort and as the airport code for County.

In your Paamiut example, the Wikipedia page you link states that the airport "replac[ed] the old heliport": presumably the  code was re-used. Wikipedia was incorrect about Chamonate: IATA's search page knows nothing about it.

BTW, the same things reproduces with the openflight database too, at least with Beaufort. Pretty hard problem to solve, I guess I'll use ICAO+IATA for unique keys.

You're right that two airports can share the same 3-letter code, but they're not necessarily IATA codes (they could actually be FAA identifiers, or just locally assigned codes).
In implementing Otto the Autopilot in our chat room, for commands like !!weather JFK, I needed to find a way to reliably convert IATA codes to ICAO codes. I ended up using the same source as you linked to, and imported the columns we needed into MySQL. A quick query shows 65 (i.e. >=130) duplicates:
AHT AJL API APS ARS AUS BJW CGY CLG CPO CSZ DDU DZN GIG HKG HYD IOR ISG IZA JFR JRS KFE KMM
KWB KYF LIM LUZ MCJ MCU MJF MNI MPD MPT MRE MUC NST NWT OBI OHE PCO PDG PHG PIV PPJ PRI PRM
PSV RCH REQ RZS SEA SFK STI SYC SZR TFY THO TNO TPJ UCN ULG VME VQS WRW YDT

The problem is, that IATA wants you to pay several thousand dollars for access to their list. So OurAirports attempts to collect all the known IATA codes, but it is user-editable, like Wikipedia. So there are inaccuracies in their database; and as I mentioned, there is some overlap between IATA/FAA/local.
The actual IATA codes are unique (although sometimes reused).

There are also IATA codes for groups of airports that serve the same city, e.g LON for LHR, LGW and STN, or BER for SXF and TXL.


text:
In terms of noise, how does a steep climb-out compare to a noise abatement procedure?
For airports in populated areas, it is common to implement noise abatement procedures. In practice, this means that aircraft reduce thrust on departure, so that noise is reduced in proximity to the airport.
However, this does mean that aircraft spend a longer time at a lower altitude, which may be bad news for populated areas further away from the airport. I am wondering how noise abatement procedures affect areas further away, up to 150km away from the airport. I would like to know how this compares quantitatively (e.g., a dB(A) chart like below, for larger distances) between procedures.

Image: Hamamci et al. (2017). Determining characteristics of lands affected by noise pollution of airports. Fresenius Environmental Bulletin. 26. 69-74.

I've not read this, but it looks like this paper would be interesting to you: https://publicapps.caa.co.uk/docs/33/ERCD9841.PDF

Some noise abatement procedures do implement a steeper than normal climb out by delaying the flap/slat retraction and acceleration phase of the departure to a higher altitude.

It’s a bit more difficult than the question suggests. Most airlines do reduced thrust takeoff not for noise but for engine life reasons. Once thrust is reduced to climb thrust, typically around 1000ft-1500ft above departure airfield, thrust will be roughly the same regardless of whether noise was a consideration or not. The main variation introduced by noise abatement profiles is when to accelerate, retract flaps and reduce thrust from takeoff setting. These are important considerations around the airfield, but at 150km there will be more difference due e.g. different weights than procedures..

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text:
Is the number of private pilots increasing or decreasing?
My interest into aviation grows by every day and I seriously start thinking about working in the industry later, as I am very interested into avionics and pilot supporting devices in the general aviation field.
However, what's the trend in general aviation at the moment? Is the number of PPL stagnating, rising or even decreasing. It would be nice if you have data about Europe, the US and International.

The statistics for the number of pilots in the US is fairly easy to come by, The Aircraft Owners and Pilots Association published a report on the number of FAA certified pilots. This shows a general static trend over the 10-or-so years preceding the report in 2011
Year    | Total Pilots
======================
2011    |   618,660
2010    |   627,588
2009    |   594,285
2008    |   613,746
2007    |   590,349
2006    |   597,109
2005    |   609,737
2004    |   618,930
2003    |   625,011
2002    |   631,762
2001    |   612,274

The graph shows the total number of pilots since 1929.

For Europe, this statistic is a little harder to come by. In general, Aviation in Europe is controlled by the European Aviation Safety Association to which pretty much all European countries are members, with a few having a working arrangement to sync up their procedures/licensing. The EU, and its associated agencies are typically bureaucracies at their best (worst!) and so there is unlikely to be a single register of the number of pilots.
For example, some elements of aviation (although NOT Licensing) is still managed by the UK Civil Aviation Authority. With all the various european and intra country agencies involved in European Aviation it will be tougher to get figures for the number of pilots.
Internationally your problem is moreso. With tens of different agencies involved in the Flight Crew Licensing, you may find it harder still to get some accurate figures.

I think plots are easier to visualize than the table, hope you don't mind the edit.

@Gypaets nope, don't mind at all. I agree with you and if I would have come across that plot I would have included it myself!

Keep in mind that the number of certificated pilots is much higher than the number of active pilots. Except for student pilot certificates, your pilot certificate never expires—even if you aren‘t flying. There are a half-dozen members of our EAA club that are in their 80s and 90s who no longer fly. And I know another bunch who got their license but haven’t used it in years. I don’t know that anyone has published an estimate of active pilots though.

@JScarry While your point is entirely right, just to nitpick my old JAR-PPL (predecessor to EASA PPL) expired. Not all licenses are lifetime.

@Jamiec I should probably preface my statement with 'In the US and some other countries..'

So, an overall 10-year growth of 1%. Compare that to the US population growth from 200-2010 of 9.7%. So a fairly substantial per capita decline


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Does cargo heat failure require a diversion? What about if there are live animals in cargo?
There was a story in the news recently that an Air Canada 787-8, enroute from Tel Aviv (TLV) to Toronto (YYZ), diverted to Frankfurt (FRA) when the pilots discovered a problem with the cargo heat.  There was a live dog in the cargo hold, and the diversion potentially saved it from freezing to death.
The narrative has been that the pilots diverted specifically in order to save the dog.  But I am wondering whether a cargo heat malfunction might require a diversion, as a matter of standard operating procedure, regardless of whether live animals are on board.  After all, even if there are no live animals, there might be other valuable temperature-sensitive cargo.
In general, according to airline policies or procedures, under what circumstances would cargo heat failure warrant a diversion?
Any major airline would be of interest; if you want a specific one, let's say Air Canada.

I would assume unless there is declared valuable cargo that requires a stable temperature above freezing (live animals, produce, etc.)  a cargo bay heater failure would not warrant a diversion - it would be unfortunate if your expensive foreign wine froze and shattered in your luggage, but that could just as easily happen when the baggage handlers use the bag as one of the pins a creative airport bowling game...

@mins: Back of the envelope: Service ceiling on the 787-8 is 43,000 feet.  Air temperature about -56 C.  Assume the cargo hold is half exposed to the outside air, half to the passenger cabin (which is held at 20 C), with equal insulation on both sides.  In the long run, we expect the temperature to approach the average of the two, i.e. -18 C.

Related: Are cargo holds pressurised these days?. See selected answer (fooot) Even in unheated cargo holds, the temperature should be above freezing.

@mins above freezing does not mean won't cause hypothermia.

Related

@mins: So it's quite possible that the fuselage is better insulated than the deck between the cabin and cargo hold.  Even so, fooot's statement is unsourced so I'd be interested to see evidence supporting it.

@mins The temperature question has been asked before although strangely - in my opinion - it was closed as off-topic

@Pondlife perhaps the question your are refering could be on travel.SE (and it is), but I agree it can be considered as ailiners' design related and thus be on-topic.

FWIW, a 777 driver I know checked his ops and indicated that they have no diversion requirement for his a/c at his company.  Ditto for a 757 driver.

Company rules normally dictate diversion/abort rules.  For most aircraft there are not temperature requirements for areas of the plane, except for the cockpit and avionics bays.  They may require non-polar temperatures to operate correctly, and may require cooling for power equipment like radar transmitters.
Obviously, if one is a carrier like FedEx and transporting live animals, it is different than transporting machined parts. A manifest for cargo will describe handling precautions, including issues with the cargo impacting the operation of the aircraft.  (eg. chemicals, radioactive isotopes, strong magnets, etc.)
Also, normally bleed air is used to heat cargo areas.  Bleed air takes energy, and is not "free" so normally the areas are not heated to temps like 20C.  Generally, if one is flying generic freight, good operating practice avoids having a cargo area which is below 0C.  Again, company rules will be where guidance is for a particular flight.

Is there ant source you can provide a link to which supports your answer?

I am familiar with FedEx manifests and some of the issues they have had transporting live animals such as orca and giraffes. Of course, in the case of animals like that, a trainer and sometimes a vet travel with the animal. The manifests have generalized handling instructions.  When there is temperature sensitive cargo, they will use an APU cart to supply the airpacks while on the ground.  There are similar crew instructions for military crews, along with cargo briefings.  I have personally been involved with those when transporting large optics as an example.

Additionally, some pharmaceuticals and medical products have tight environmental parameters, and the manifests reflect that.  Things like human and animal organs are packaged to stabilize the temperatures, but that packaging has practical limits.

Is there ant source you can provide a link to which supports your answer? – Ralph J Apr 2 at 20:34    Which part of the answer are you seeking verification about?

@RalphJ, what part of the answer are you looking for a source? Company procedures? The absence of regulations regarding cargo hold temperatures in the US? Or perhaps the energy cost of bleed air?

Exclusive Cargo airlines are expected to take extreme care of temperature sensitive cargo (pharmaceutical products, food, animals, etc.).  Modern cargo aircraft have complex air-conditioning systems not less sophisticated than passenger compartments and special briefing to operating pilots.
Cargo airline company procedures will provide clear guidelines to pilots under what circumstances would cargo heat failure warrant a diversion. This will depend on the ethical nature /and/ cost of damage due to damaged cargo.
(Example:  Is the cargo in question, thousands of a day old chicks /or/ a few expensive racing horses)

Welcome to aviation.SE! please be aware that if possible we like to have references, do you have any for your statements? for example, about these guidelines, do you have any example?

Common sense applies. The regs don't require a diversion for a cargo heat fail. But, if you know there are live animals down there, and you know it's -53 outside, then you're going to divert to save them.
As the other poster said, if it's just some expensive something else down there, then no.

do you have any source for "The regs don't require a diversion for a cargo heat fail"?

Common sense does not hold in front of procedures. Procedures are designed according several decades of experience to insure good handling of many normal and abnormal situations. Heat failure does not mean the cargo's temperature will immediatly goes down to negative temperatures (insulation, airflow from the rest of presurized area,...). The diverstion decision must take into account many other elements that must be mentionned on procedure and forgotten by common sense.


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What is the difference between "flight level" and "altitude"?
I have seen references to "FL180" and "FL300", and I know that they stand for Flight Level 180 and Flight Level 300. I've also seen references to "an altitude of 18000ft" or "an altitude of 30000ft".
What is the difference between flight level and altitude, how do they relate to each other, and when and why would one be used instead of the other?

Not mentioned in the answers, but there is no such thing as an altitude of 30000ft.  It would only ever be planned, flown and reported as a flight level.  Altitude only works if everyone is using the same pressure setting, which is what flight levels achieve since everyone is using the standard pressure setting.  Under IFR, you are required to switch to flight levels when ascending through the transition level.

But there is, @Simon, from the non-pilot, layman's perspective (mine), and that's why I asked the question.

I understand that, and was not directing the comment at you.  One often sees altitude misused in uninformed comments.  Now you know how to read it correctly ;)

@Simon, in military flying you absolutely might work at an altitude of 30,000' -- for instance if you're dropping things and you want to be at a computed height above your target.  That requires shifting the altimeter over to a QNH that the weather guys give you for the target area, which you'd probably do shortly after "going tactical," so that adherence to normal IFR rules is discarded in favor of doing what it takes to accomplish the mission and be a hard target for bad guys shooting at you.

@Simon According to Casey's answer below, Nepal would have altitude of 30,000 ft. Mount Everest is there.

@Koyovis I don't understand your logic, or what point you are trying to  make.

@RalphJ 4 years on :-) - One usually wants to avoid being a hard target :-)

Flight levels use QNE or pressure altitude, while altitude references QNH or local pressure adjusted to sea level pressure.  Altitudes are used at low levels and flight levels at higher levels.  The transition between altitudes and flight levels differs by country and is generally just above the highest obstacle in that country.  In the US the transition altitude/level is 18,000' / FL180.  Some countries transition as low as 5000' / FL050 and the transition altitude/level may vary from airport to airport.
In the altitudes knowing accurate elevations relative to the ground and obstacles is important for collision avoidance and this is the reason QNH is used here.  Each airport will report QNH and controllers will issue the current QNH as needed.  You need to know the QNH for obstacle / terrain avoidance but you need to be using the same QNH as those around you for aircraft vertical separation.
Above all terrain/obstacles the only thing we care about is vertical separation, so we no longer need to know about the actual pressure and instead use a standard reference pressure, QNE / 1013.25 hPa / 29.92" Hg.
Note that flight levels drop the last two zeros of the corresponding altitude and so 30,000 is FL300, not FL30000.  When checking in with a controller, FL300 would be pronounced flight level tree zero zero.
It is also worth noting that an altimeter cannot actually determine altitude.  It can only determine pressure (technically local static pressure compared to a reference pressure).  It converts this pressure to an altitude using a calibrated non-linear scale.  To illustrate the point, look at this map of 500 mb heights:

500 mb correlates to 5500 m or 18,000 ft in a standard atmosphere.  In a real atmosphere this height varies and is not actually level.  An airplane flying "level" at FL180 from LAX to NYC last night will have actually descended almost 300 m while indicating a constant altitude.  These deviations in true altitude from indicated altitude are acceptable, however, since they effect everyone equally in the same locality and separation is maintained.

@SentryRaven NATO phonetic alphabet specifies the pronunciation of 3 as TREE. I know you already know this. :)

@Farhan the 3-year-old handbook also specifies the pronunciation of 3 as TREE. You would only know this if you associate regularly with tree-year-olds. ;)

"These deviations in true altitude from indicated altitude are acceptable, however, since they effect everyone equally in the same locality and separation is maintained." While I understood your explanation, this was the part that really sold me on this as the 'official' answer.

@FreeMan, not only is it acceptable but it is actually preferable, because it eliminates the need to periodically adjust the altimeter and the associated risk of error in doing it.

Funny, casey, after 2 years, I just reread this and realized your example flight is from "LAX" (airport code) to "NYC" (a city), not JFK, LGA or NWK...  I've learned a lot in those 2 years! :)

And 4 years later, I realized that Newark Airport is EWR, not NWK.

If the U.S. transition altitude is 18 kft, then what do pilots flying in the vicinity of Denali (peak altitude 20.32 kft) do?

@Sean There are special rules around Denali, which is worthy of its own question.

FL180 and FL300 stand for Flight Level 180 and Fight Level 300.
Flight levels are spaced 100ft apart on an altimeter that is set to the standard sea level pressure (QNE) of 1013.25 hectopascals or 29.92 inches of Mercury. So indeed, FL300 means 30,000 ft.
Altitude 18000 means that the altimeter indicates 18,000 feet and that the altimeter is set to the QNH, which is the pressure reading on the ground corrected to sea level pressure using the standard atmosphere.
When there is a low pressure area, the QNH will be lower then 1013.25 hPa. When you compare two altimeters; the first set to QNH < 1013.25 hPa and the second one is set to the standard setting of 1013.25, the first altimeter will indicate a lower value than the second altimeter.
QNH setting is used at lower altitude where obstacle and terrain clearance are important. But for long distance traffic it is a nuisance to change the altimeter setting as the aircraft flies through different pressure areas on the ground. Therefor the Flight Level concept was introduced, allowing everybody on higher altitude to use the same setting. This also reduces the chance that aircraft have a different altimeter setting in the same airspace, which would cause vertical separation problems.

Note, there's also FL3000M which is to say 3000 meters using standard pressure.

To ensure I understand: 1) the altimeter reading will vary based on the local air pressure in addition to the actual altitude 2) to accommodate that, FL was invented and is based on a standardized setting. Correct?

@FreeMan Flightlevel were introduced to maintain vertical separation between aircraft enroute, without having the aircraft change their altimeter settings to local QNH every few miles. Two aircraft at FL200 and FL210 flying close to each other will be 1000ft vertically separated, irrespective of how the actual pressure around them changes, as both will be subject to the change. This is why there is Transition Altitudes, Transition Layers and Transition Levels, so pilots know when to change from QNH to standard pressure settings.

@FreeMan The altimeter reading is basically a pressure reading which is converted to altitude using a standard atmosphere model. In order to ensure that the altimeter reads the airport elevation at the airport, there is a setting that corrects for the actual pressure at the airport. There tow factors directly affect the altimeter reading: 1. the ambient (static) pressure around the aircraft, 2 the pressure setting of the altimeter. The ambient pressure depends on height, pressure at sea level and temperature gradient in the air.

You should add that this is valid for FAA regions, but ICAO for example has different transition altitudes.

@Maverick283 ??? This answer is generally applicable. I don't even mention the concept of transition altitude.

Alright sorry, didn't read that tooo good there, but I was referring to the higher altitudes. So my suggestion were just for completion of your answer.

@Maverick283 Ah, I see, it was the 18000 ft you see as being FAA typical.  I agree with that, but I just took it for the question. In Europe we generally have much lower flight levels. Here in the Netherlands, which is pretty flat country, the transition altitude is 3000 ft AMSL

Altitude
The vertical distance of an object measured from mean sea level.
Flight Level
To understand a flight level, we should understand how altitude is measured in an altimeter, which is essentially a calibrated barometer - it measures air pressure, which decreases with increasing altitude. To display correct altitude, a pilot re-calibrates1 the altimeter from time to time, according to local air pressure.
Flight levels solve this problem by defining altitudes based on a standard pressure of 1013.2 mb (29.92 inches Hg). All aircraft operating on flight levels calibrate to this same standard setting regardless of the actual sea level pressure. Flight levels are then assigned a number which is the apparent altitude ("pressure altitude") to the nearest thousand feet, divided by one hundred. Therefore an apparent altitude of 18,000 feet is referred to as Flight Level 180. Note that aircraft may be at some other actual height than 18,000 feet, but since they all agree on a standard pressure, no collision risk arises.
Flight levels are not used close to the ground, for perhaps obvious reasons - obstacles are fixed to the ground and so their absolute height needs to be known. The altitude of the lowest flight level varies from country.

1: Re-calibration of altimeter is done to avoid airplanes flying at the same height, though their altimeters show different altitudes. This is safety issue.

Concerning the altitude section, isn't "mean sea level" defined as 29.92 inHg? Otherwise it's not mean sea level, it's the actual sea level, isn't it?

@Gio Mean Sea Level is the local sea level given as the average between high tide and low tide. The pressure at that point may or may not be 29.92" Hg, depending on atmospheric conditions.


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What is the teardrop-shaped appendage on the T-6 Texan?

Can anyone explain what is the teardrop-shaped item behind the cockpit?

Clearly it's not on all models. this one has it but this one does not and this one has something different but in a similar position

It is the RDF antenna 'bullet' (or teardrop, actually) fairing. This image shows the (loop) antenna itself.

Source: mission4today.com
Initailly, RDF antennas were circular loops mounted above or below the fuselage.

"Earhart-electra 10" by Originally uploaded by w:User:Logawi - Transferred from en.wikipedia, description page is/was here.. Licensed under Public Domain via Commons.
Later loop antenna designs were enclosed in an aerodynamic, teardrop-shaped fairing.

Source: airpages.ru

The change, IIRC, is that the RDFs evolved into ADFs, which rotated to find the relative angle from the nose at which the beacon signal was strongest, allowing the pilot to get a fix from the beacon without having to change course or manually manipulate the antenna. The motorized rotating antenna, however, was much less aerodynamic and prone to damage at high airspeed, so it needed a housing to contain it.

authors: فرهاد میرزایی


text:
Why don't combat aircraft have rear-facing weapons?
This excellent question addresses the role of dogfighting in modern aerial warfare. However it seems that most ACM concentrate on maneuvering into shooting position. Why don't combat aircraft have rear-facing weaponry?
Even WW-II bombers had rear-, side-, and downward cannons, albeit manned. Could a small electronically-controlled cannon be added to the stern dorsal part of a combat aircraft, for shooting rearward? I've personally seen remote-operated FN MAGS on vehicles, the whole system seems to weigh on the order of 100-150 KG including hundreds of rounds. The vehicle-mounted systems took up about a cubic meter or so of space, but a purposed-designed system could probably take up less than half that, and probably shave off a good portion of the weight as well.
Could a computer not point a barrel at a vehicle with a relative velocity approaching 0 and shoot, even in a +Mach 1 wind in the direction of the target? It may be enough to just get a few bullets in, shotgun style. The Uzi rifles famously don't shoot straight, they make a cone of bullets. Perhaps this cone, if aimed in the general direction of an aggressive aircraft, might be enough to either disable it or get it off your tail. A short barrel will make a wider cone, perhaps the rear-facing weapon could have an very short barrel. ACMs such as the Cobra maneuver seem to imply that there are a finite number of angles from which attack is most likely, the weapon could be limited in angular range to those angles.
I'm sure that piloting an aircraft while trying to avoid being shot is difficult enough, so I wouldn't assume that such a system would be of much help on a single-seat aircraft. However, in a dogfight presumably the ROI or navigator could operate the guns while the pilot does the aviation.

Have you ever played a computer game with a rear-facing weapon? Complete waste of upgrade points. :-)

@DavidRicherby: Actually, I haven't. Perhaps a computer could do better. Computers control fire in the Korean DMZ, for instance.

I wonder why you don't mention missiles?  It seems like launching a short range missile would be fairly easy (in my head), and it would certainly circumvent the recoil problem (just drop it before it fires).  I only ask because the question title says "weapons" and the question mostly deals with the guns.

@JayCarr: I discuss guns as I'm more familiar with them and they are simpler. If you can address the issue with missiles, please do!

Who control the rear gun?

@JayCarr: Modern off-boresight missiles can actually fire at an enemy behind the aircraft. First developed by the Russians, off-boresight targeting systems only requires the pilot to turn his head to look at the aircraft behind him for lock and then the missile will fly in a circle to hit the target.

It seems like a great idea, doesn't it? But it'd have very short range, very low Ph, very low Pk, high weight per unit of lethality. Aircraft cannon and rounds are necessarily large. The weight could be better spend (eg) on super-maneuverable missiles that can almost fire behind you, which is basically a modern, short-range, IR dogfighting missile (eg, AIM-9X). Another concept are defensive mini-missiles that hit aircraft and incoming missiles. Another USAF program is a tiny defensive missiles fired from the chaff/flare dispensers.

Machine guns were fairly effective weapons in WWII and shortly after. However, this was only because the attacking aircraft were also armed with machine guns. Even then, the fighter aircraft could be effective against not only the armed bombers, but also against the fighter escorts.
Bombers of the time did typically have tail gunners. However, this only resulted in a change of tactics, with fighters attacking from a different angle. Defending against one means of attack just means the attackers may choose a different attack vector.
However, the rocket age changed all this. Modern missiles can be fire-and-forget, automatically tracking a target from many miles away. Guns aren't very effective in this type of combat. In fact, it's debated whether fighter aircraft even need a forward facing gun anymore. The gun on the F-35 may not even be usable until after the first squadrons are operational.
At high speeds, the rear-facing gun would have to overcome the forward velocity of the aircraft, decreasing its effectiveness.
One could imagine something like a CIWS being helpful, but the weight and size would not justify the benefit for being included on a fighter, or even a bomber. Modern aircraft have chosen electronic defenses over mechanical ones. These can be lighter, easier to integrate, and are more effective in a wide variety of situations.
However, combining the two ideas and arming aircraft with lasers is still a possibility for the future.

A small, rear-facing aircraft-designed device with a purpose similar to a CIWS is quite what I had in mind. Thank you.

Slightly off topic, but... RE: aerial missile combat & the F35. Hasn't the USAF learned anything from Vietnam? The A6, F4 and others of that era weren't equipped with any guns because "missiles made them obsolete". A poor decision many paid dearly for.

@FreeMan: There's such a thing as overcorrection. While missiles weren't good enough in Vietnam (especially since battlefield awareness and reliability were too low) 50 years of progress might very well have fixed that. With AMRAAM and AWACS, BVR kills became a reality in the 90's.

The unreliability of the missiles was only half the problem; rules of engagement often forbid firing missiles without visual identification of the target, which makes even reliable medium-range missiles worthless.

Don't missiles have a minimum range? I don't think they would be effective in a dogfight (as much as the entire machinery is designed to avoid these).

"At high speeds, the rear-facing gun would have to overcome the forward velocity of the aircraft, decreasing its effectiveness." -- while that's true of course, if the following aircraft is at a similar velocity to the firing aircraft, the relative speed of the bullet would still be as high. Possibly higher, as bullet would experience less aerodynamic drag due to a relative "tail wind" of 500mph.

RE: The "may not need ... gun" link.  Corollary to "why a gun?" is  disproportionate application - for example wiping out a sampan with a million dollar missile.

@MSalters. It‘s all about resilience. Be always able to use a plain approach. What you do if the others have radar jammers and you too? Not talking about just have no missiles anymore. And you can fire „warning shots“ or crazy things like just try „only“ damaging something.

If your opponent is directly behind you, you have lost the fight. The point of basic fighter maneuvering, or BFM, is to keep your opponent off your tail and hopefully get on his. Everything you would learn as a fighter pilot centers around ensuring you would never have to use a rear-firing weapon.
In addition, visibility directly behind the pilot is typically poor; humans can't turn our heads 180 degrees, and if we could we'd be staring at the headrest behind us. A few fighters have had rear-view mirrors, typically ones with a two-piece canopy including the Eurofighter, F-15C and F-14 among others, but the one-piece bubble canopy on fighters like the F-16 and F-22 give no good place to put a mirror that wouldn't further interfere with the pilot's forward vision. Rear-view cameras would be laughed at by pilots as a waste of an MFD.
A cannon firing backward would also be a pretty single-purpose weapon; in a multirole fighter you might get a chance to use it to add insult to injury when pulling out of a ground run, but in all likelihood you'll either have peeled off to one side as soon as you pickled your bombs, or you'll be toss-bombing and not even overfly the target. The only real reason you'd ever fire it would be to keep someone directly behind you on their toes.

If your opponent is directly behind you, you have lost the fight. That is exactly why (I think) that rear-facing weapons are needed. And the device would be as single-purpose as the ejection seat and other safety systems are.

@dotancohen Adding an extra 50kg of weight is a significant tip in the wrong direction in determining which plane gets on whose tail, and wins. Having two guns in different directions splits your focus, and has worked best for less maneuverable planes such as bombers. If you've got such good computer targeting that a computer tailgun would have you win that, then you might consider a turret mount instead.

I'd like to note that modern off-boresight missile systems can basically fire behind the aircraft by having the missile make a U-turn after launch. Typically they can fire up to 120 degrees but for short distances some can turn 180 degrees to shoot straight behind (the more you need to turn the more fuel you need to burn so the less range you have)

@slebetman - The only instance of targeting an aircraft behind the shooter that I know of required a second aircraft to illuminate the target and transmit the information over a datalink to the firing aircraft. The missile, a MICA IR, then simply launched at maximum pitch rate and "locked on after launch". So, possible? Yes, but as to how useful it would be in a dogfight remains to be seen.

@KeithS: Current ASEA radar can now scan up to 120 degrees to the side - basically up to 30 degrees to the back. Current off-boresight firing systems allow radar information to be uploaded to the missile immediately before firing so that the missile doesn't need to "see" the target until it's turned around. Still, to the best of my knowledge, while both capabilities exist I'm unaware of a live test firing of a missile beyond 90 degrees. I believe there was a Russian demonstration of firing at 90 degrees which scared the s**t out of NATO some time back.

So the status is currently "plausible" but maybe not tested. The 180 degrees figure is a quote from an interview with a test pilot and I believe that's more about the maneuverability of the missiles themselves, not the targeting system.

Still, not needing to point the nose of the aircraft to your target to shoot him down is a dramatic change in tactics. You no longer need to be behind him, you just need to bank and yank a bit so that he's within your 120 degree field of fire.

Yes, and this was a frightening revelation to NATO forces in the years after the USSR collapse, when Western forces got their first close looks at the hardware left behind in Warsaw Pact nations. The AA-11 Archer was known to be a threat, but U.S. top brass didn't know just how outmatched they were until they watched mock dogfights with East German pilots. This new understanding led directly to the JHMCS and the next generation of NATO IR missiles like the AIM-9X, ASRAAM and MICA.

You can't have a camera system that tells you when something interesting happens and that occupies zero screen space the rest of the time?

When in a dogfight, you cannot shoot straight ahead without moving into a position which anticipates where your adversary will be when the bullets hit him. This means you need to turn tighter, to get into his circle. When both aircraft have similar specs, this is harder than it sounds.
The Russians once tested an idea which looked obvious at first: Svivel the gun upward, so it shoots into the circle without the aircraft needing to maneuver into the circle itself. They tested this on a MiG-15, but the results were disappointing. As soon as the gun was misaligned with the fuselage by more than a few degrees, the recoil from the gun would cause the MiG to pitch down; the last thing you need in a dogfight.
The lesson is easy: Shooting at an angle from a point far away from the center of gravity will cause control issues and might even be a greater danger to yourself than to your adversary. It might be that a modern FCS can compensate for this, but to my knowledge this has never been tried.

But a gun pointing backwards will also shoot in line with the CoM like the forward facing guns.

@ratchetfreak: In a dogfight the rear-pointing gun will have to shoot into the circle just like the forward-facing gun. Only when the plane is ambushed in straight flight will the gun point along the longitudinal axis, but then I expect the pilot to be unaware of the threat.

It might be worth mentioning that the cannon on most aircraft actually does point slightly upward, giving the shells a parabolic arc to increase useful range. It's only 2-3 degrees in most cases though.

@KeithS: Correct, but it is not for range increase, but for making the bullets hit at the expected firing distance. The MiG-15 had three guns, one 37mm monster and two 23mm for smaller targets. They were designed to hit a B-29 at several hundred metes, and the higher nozzle velocity of the 37mm gun necessitated a shallower angle. When shooting at a F-86 from a much closer distance, the 23mm rounds would fly over and the 37mm rounds under the target. That is why their kill ratio was so poor.

I'm actually talking about newer aircraft; the F-16's Vulcan cannon is canted upward about 3 degrees from the stable "velocity vector" of the plane in clean configuration. This upward cant does produce an optimal range (something like 1500 feet), but the plane has some pretty sophisticated targeting modes including a truly ingenious "funnel" mode that account for range even without a radar lock, and with the plane having only one cannon, getting several different guns to hit the same place isn't a concern.

The real concern is being able to adequately lead the target in close quarters: that 3* cant puts the shells up into the top of the pilot's HUD in level flight, so when he's in a tight-turning furball, he doesn't have to point his nose as far in front of the target to hit him. First, getting that far in front of an enemy is difficult when that pilot doesn't want you to do so, second, getting that far ahead can cause you to lose your target literally under your nose, and third, getting that far ahead could lead to you ending up in front of him, which is not a good place to be.

Dismissing missiles for a moment, let's focus on fixed guns.
Rear-firing cannons simply make no tactical sense.
If you have an enemy fighter on your six, and in gun range, you need to pitch, roll, circle, and generally maneuver like crazy to throw off his lead aim to avoid getting shot out of the sky.
The very last thing you want to do is to fly in a smooth line lining him up in your (rearward) sights while he blasts you with his (forward) cannons.
That is not even taking into account that aiming the back of an aircraft at a (maneuvering) target behind you is an order of magnitude more difficult than aiming the front of the aircraft at a target in front of you.

So, fixed guns are out. What about remote-controlled, swivel-mounted guns?
This has actually been tried as early as in the Me 210. You still face the issue of wasting weight and space on a weapon that can only fire at the enemy if he's right where you do not want him in the first place. Speed, stealth, diversion, all these increase your chances of survival much more than pitting your rear-facing weapons against a fighter's forward-facing weapons. Other than the Me-210, Me-410, and prototype versions of the Arado 234, I don't know of any aircraft that attempted remote-controlled rear-firing turrets.

So, bury the guns. At which point I'll say that I don't know enough about modern missile technology to say if modern air-air missles can or cannot actually engage targets to the rear, and bow out of the question.

Older version of the B-52 had a tail gun turret too

A bit late to the party but someone just commented on this so it popped back up.
The answers given don’t really address the real reasons why rear-facing guns aren’t a sound investment any more.  Military design is almost always about he mission ecosystem in which something will be deployed, and no one has really addressed this.
There was a time when the only way to blow up a target was to fly a B-17 over the target.  And you were going to have to fly that B-17 over that target irrespective of how much anti-aircraft fire, or how many fighters you were going to face.  You went anyway.  And so bombers really needed 360 degree defensive firing positions.
The super-power Air Force of today is much less equipped for operations in contested airspace because that isn’t really our MO any more.  We have a lot of ways to blow up the target that no longer involve flying a B-52 deep into the threat envelope of contested airspace.  And thus we aren’t going to fly a B-52 into any situation where an enemy fighter can get behind it.  Because we’re not going to be flying that B-52 in a situation where there’s enemy fighters anywhere.
If you see a B-52 flying over you, the bad news isn’t just that you might get a bomb dropped on you.  It’s that seeing that B-52 means that your air defenses have already been destroyed, first by cruise missiles, then SEAD missions with anti-radiation missiles, then predator drones.  It means your airfields and runways have all been destroyed.  It means your enemy already has patriot batteries in place, is already flying close air support, etc.  It means well over a few billion dollars of weapons have already completely dominated your airspace in every way.  Otherwise, there wouldn’t be a B-52 flying overhead.
My point is that the reason that most aircraft no longer sport rear-firing weapons is not because rear-firing weapons are to hard to aim or are impractical.  It’s because most NATO aircraft are designed to operate within an mission set where there won’t, and can’t, be an enemy aircraft behind you.  Why put a rear-facing gun on an airplane that we never intend to deploy in situations where there’d be someone behind you to shoot at?

That is an interesting perspective. The B-52 is fielded only by the United States so far as I know, which does have the airspace-denial capabilities that you mention. So this is a good argument for the B-52 specifically. But e.g. the F-35 is fielded by many nations which do not have airspace-denial capabilities, and may very well find themselves in contested airspace, as the current Ukranian-Russian conflict demonstrates.

@dotancohen (a) The F35 is not a dogfighter.  The single design criteria for the F35 is standoff.  It can detect your presence from further away than you can detect it, and its weapons have the range to engage you before you can detect its presence.  They are spending billions for the standoff, not for its dogfighting.  And it has both an organic sensor platform, and operates in sensor-rich environments to ensure the F35 will know about you before you know about it.  (b) Singapore and Japan don’t have any illusions about fending off China alone.  But their investments show they are committing…

…to a geo-political sphere of influence.  They are “tied” to western defensive strategy even if not by formal treaty.  (c) None of those countries are only buying F35s.  The F35s are part of investments they are making in other defense modernizations like air defense, drones, etc.  (d) Look at the F15 track record.  It has been used in air combat and no one has ever successfully gotten behind one to attack it.  All who have tried, without a single exception, were already dead well before they got close enough to be engaged by a rear-facing cannon.

Your description of the situation could be interpreted as Singapore and Japan funding and staffing US fighter squadrons under the illusion of having their own air force.

Bear in mind this is all speculation:
Firing to the rear could be a bit optimistic; the pursuer won't be straight behind, he may be anywhere in a wide cone behind.  (How wide that cone might be, we'd need to ask someone with experience in this area!)
A remotely swivelled cannon would probably be too slow to point anywhere within that cone, as the pursuer will naturally fire as soon as he has a position.  Some heavy bombers have cannon (remote or manned) to the rear, but these are unlikely to be used in fast dogfights.
Also note most recent dogfights seem to have involved missiles, see gulf war engagements list.  The preferred use for cannon on fighters might be in the ability to use a non-lethal show of force against heavy bombers or spyplanes, in a cold war situation.  I recently found this mentioned while reading up about the old Lightning (search that link for "warning shots").

How the use of cannon is non-lethal? Even tracer round is lethal if hit the right place.

@vasin1987: It's not lethal if you miss. Think "shot across the bow".

@FredLarson everything is not lethal if you aim to miss. Isn't the presence of escort fighter enough to prove a point more than the provocative firing a canon?

@vasin1987 Not necessarily. These confrontations can be a lot more complex than "we have a fighter, so we'll shoot you if you don't obey, so you will obey." Deploying an escort fighter doesn't mean you're willing to take actual action (and create an international incident), so it also doesn't mean the other side will immediately obey. Since actually shooting down an aircraft is typically the last resort, warning shots add a "we are very serious" factor; they aren't something to do lightly, and they are more effective than just a fighter in convincing someone to obey.

As for missiles, you can't use missiles as warning shots, because it's much less clear that they aren't actually an attack (missiles having homing capability and all). It's the exact same principle as firing shots across a ship's bow.

Rear launching missile might be possible, but I think if you have the option of adding another missile, I imagine most pilots would prefer to have it forward facing so they can use it to aggressively attack and destroy an opponent. Dogfight tactics traditionally favour aggressive offensive tactics as opposed to defensive ones, and therefore forward pointing weapons makes more sense. If your enemy is behind you, you've already screwed up.
However the French actually managed to destroy a target behind the launching aircraft with a MICA missile (unsure if it was the radar guided or IR guided variant)
https://en.wikipedia.org/wiki/MICA_(missile)#cite_note-4
Unfortunately the original source is a dead link.

If by combat aircraft you mean fighter jets; then the answer is simply:

If the opponent is behind you, you have lost the battle.

The entire point of aircraft with extremely mobility and flight dynamics is to keep the other guy in front - where all the guns are pointing.
I am not too familiar with fighter aircraft radar and targeting systems, but in most modern fire and forget systems, once the target is acquired, the projectile can travel in any which direction towards it destination. This greatly reduces the need for "rear facing" weapons since the weapon is independent of the aircraft.
There are combat aircraft (non-fighter jets) that have weapons (usually, a gun) on the front, top, sides and yes - even rear.

Some tactical fighter aircraft can be armed with rearward firing weapons. The Russian SPPU-6 gun pod containing the six barrel GSh 6-23 cannon can be mounted on a pylon facing either forwards or backward and is used for defence suppression. This pod allows movement of the cannon through 45 degrees in elevation and 45 degrees to either side. There is also an SPPU-22 pod which mounts the two barrel GSh 23 cannon, trainable in elevation only and which can also be fitted facing forwards or backwards. These pods can be carried by most current Russian tactical aircraft.


text:
Backup airspeed indicators (lack thereof)
This recent aviation.SE question asks why isn't GPS used as a backup speed indicator. The answers given mention that GPS cannot indicate airspeed. However, there are other methods to indicate airspeed. At the very least, a small anemometer and vane might be deployed in a fashion similar to how a ram air turbine is deployed. Or at the very least, deploy the ram air turbine and measure the RPM or voltage.
Another creative method might be to release a small red sponge ball, and have a camera/sensor mounted in the top of the tail (where the A380 already has a camera) determine the vector that it flys away. Image processing for this type of work has been available for years already.
Of course, these solutions would only be deployed when necessary. I wager that using the ram air turbine to determine the airspeed could be done in software with no added weight or mechanical components added to the vehicle. Assuming that the vehicle already has a camera in the tail (such as the A380), adding the ability to deploy a small sponge ball would be less of a retrofit than adding cellular telephone service.

Even better is a venturi http://www.eaa.ca/experimenter/images/0906_venturi_sm.jpg

Both of the methods you propose have significant difficulties.  They would need more information from other systems.  For example, the RAT will rotate at different speeds for the same airspeed and different altitudes and on different days.

There is a much simpler method than any you've proposed:  If you are flying safely, and you lose airspeed indication, leave the pitch and power where they are, and you will continue to fly safely.

@Simon: The premise of the question is that the pitot tubes are inoperative. Thus, the plane is not flying safely (the crew cannot know the true airspeed). They need to know right now what their true airspeed is this second.

we already have backup procedures for a broken ASI, which is to fly pitch and power. the problem is not the lack of a airspeed reading, but poor airmanship.

I repeat.  If it was flying safely before the ASI failed, why cannot it continue to do so?  Same pitch and power = same airspeed.  I am of course assuming cruise, otherwise, this and your methods all fail.

"So, how does one shoot an approach with no altimeter, airspeed indicator or VSI? It's not really as hard as it may seem. Remember being taught that pitch plus power equals performance? Well, I'm here to tell you how well that works."  http://www.avweb.com/news/airman/184279-1.html?redirected=1

Some aircraft do have backup ASIs. For example, one of the Tiger Moths I fly has a hinged plate on the wing struts that rotates as the air pushes against it, and you can read that angle against a track marked in mph.

@Simon: Ask Air France Flight 447.

That is precisely the case study of what happens when pilots do not use good airmanship and correct decision making.  If they had left pitch and power where they were, they would still be here today.  There is no reason that AF447 could not have been safely recovered.

They need to know right now what their true airspeed is this second..  Why?  It was mach 0.82 the second before the failure.  It is mach 0.82 the second after the failure.  Your ball tells them that in the second in between, it is mach 0.82.  What next?

So what happens when they come out of cruise?

In order to answer your question first we need to figure out what we're trying to accomplish with airspeed. For the most part the goal is "envelope protection" - keeping the pilots from either stalling the aircraft or overspeeding (and overstressing) it.
So the first thing to know is thaton modern transport category aircraft there are already backup systems to compensate for a failed airspeed indicator.
For starters there are usually at TWO (visible) airspeed indicators (one for each flight control station - pilot & co-pilot). The airspeed indicators are usually using two different pitot (ram air pressure) sources, so that if one pitot system or indicator fails the aircraft can still be flown on the others.
In addition to those two indicators there is often a third "tie-breaker" system on modern aircraft which is used to determine which system has failed (if two systems agree the third is declared insane and ignored).
In the unlikely event that all of the pitot-based airspeed indicators fail they aren't the only system providing envelope protection: Angle-of-Attack vanes are used to determine when the wing is approaching the critical angle of attack (and is in danger of stalling). These operate independently of the pitot system as a redundant form of stall protection, and as with the pitot system aircraft usually have more than one AoA vane installed.
Both the pitot-based airspeed indicators and the AoA vanes are critical systems, and are heated (to prevent ice from accumulating and interfering with their proper operation) and monitored (by onboard computers and/or the pilots).
Finally in the face of a failure of all of those systems there is the ultimate fallback method that others have described: Pitch + Power = Performance.
A pilot should be able to at the very least maintain level flight, descend, climb, and make standard-rate turns within a safe flight envelope by reference to engine power and aircraft attitude.
While that's far from an ideal situation it is one pilots are trained for.

Thank you! This answer does seem to address the concerns, both my own and those of others, in the comments on the OP.

Is it right that for getting a fairly good landing speed, if you declare emergency, the tower should be able to estimate your KIAS based on radar data and whether?

@yo' It's possible, but it would be the same quality estimate you'd get from a GPS and a wind report/forecast: If you know the wind you can work out your airspeed given your groundspeed.

To expand on voretaq7's answer -- airspeed is measured by aircraft and used by pilots as a proxy for Angle of Attack because early aircraft designers found it easier to measure airspeed (using pitot and static pressures in a calibrated differential pressure gauge aka airspeed indicator) than they did measuring the Angle of Attack (alpha) of the wing directly (this requires an AoA vane with its associated electrical transducer).
The US Navy has a long-standing practice of fitting AoA gauges into the cockpit of its aircraft, and naval aviators are trained to use the AoA gauge as a primary flight instrument; however, civil aviation retains the legacy of airspeed as a proxy for AoA, despite the near-universal fitment of AoA vanes to transport class aircraft to provide stall warnings.  This has started to change, though: Boeing glass cockpits with recent firmware provide a mini-AoA gauge as an option.  While it's not accurate enough (primarily due to annoying Mach effects) to be used as a primary flight instrument in the style a naval aviator uses their Hornet's AoA gauge when landing on a carrier deck, it's still useful as a backup/crosscheck for airspeed trends (if your ASI is showing 50 knots with the AoA indicator reading low, you have a broken ASI, not a stall).

This is a very good observation. I think it would help with learning as well. I used to fly an aircraft with automatic slats and I think the subconscious feedback when they deployed helped me a lot. A angle of attack indicator would be much nicer.

Possible approaches for a backup Airspeed Indicator
Why not use GPS?

Does not indicate airspeed, only ground speed.

Why not use the RAT?

The RAT will rotate at different speeds for the same airspeed and different altitudes and on different days. (Simon)

Why not use a coloured ball?

The ball would be straight back during cruise unless weighted down significantly. (ratchet freak)
The speed of the ball would help determine airspeed. For a second or two. Now what? And there is a much simpler method. If you are flying safely, and you lose airspeed indication, leave the pitch and power where they are, and you will continue to fly safely. (Simon)

Why use anything at all?

We already have backup procedures for a broken ASI, which is to fly pitch and power. the problem is not the lack of a airspeed reading, but poor airmanship. (rbp)
If the plane was flying safely before the ASI failed, why cannot it continue to do so? Same pitch and power = same airspeed. I am of course assuming cruise, otherwise, this and your methods all fail. (Simon)

Other approaches:

Some experimental aircraft use a venturi  to measure airspeed. (rbp)
The Tiger Moth has a hinged plate on the wing struts that rotates as the air pushes against it, and you can read that angle against a track marked in mph. (Dan Hulme)

I should add that it is also possible to fly pitch and power in regimes other than cruise, it's just a lot more complex and requires knowledge of the numbers.  In cruise, leave it all alone and it will continue to cruise.


text:
Are the airport "taxi" plans available online?
Whenever I sit in an aircraft and we taxi around the airport, I wonder how exactly is the airport organized, for example, the arrangement of the taxiways and runways.
Is it possible to locate airport diagrams? I'm mostly interested in Paris CDG, Prague PRG and Amsterdam AMS.

See this question: Is there an online source for European Instrument Approach Procedures? which would include taxi diagrams like you are asking about.

Perhaps an interesting destination in that general area is also Austria, charts are available at VACC Austria.

A great site to download airport diagrams and other charts is charts.aero. As of April 2015, however, the site seems gone.
Airport diagrams show all the taxiways and ramps, which is what you are looking for.
Keep in mind that this is an unofficial website. The charts should not be used for actual navigation.

Hmm, I don't see any charts for EHAM at the link you've provided... I'm accessing from the USA.

@JeffBridgman Same here. It may be a temporary issue on the site. I'll keep an eye on it and if it doesn't fix itself I will mention that in the answer.

@JeffBridgman: Apparently there was some legal problem with European data. It seems to be resolved now, but the URLs changed. Now it is http://charts.aero/?icao=EHAM, or just use the search.

@PhilippeLeybaert It's still down as of April 2015

In the US, you can find this on the FAA's website here. You can search for a US Airport, and it will pull up a Airport diagram with Taxiways, Runup areas, and Runways on it.
I couldn't find an official guide for European airports, but this site has a detailed guide for CDG, among others, with a taxiway map being one component. They also have a similar guide for AMS. They do not have one for PRG. Keep in mind these are unofficial charts.

As Lnafziger noted, this site also has airport charts, including approaches. The downside is you must register (free) first to access them. You can read more in this answer.

The only place I could find PRG charts was at the site Phillipe linked to. They are here. The site Lnafziger may have them, but I haven't registered to find out yet

Yes, official airport charts for Europe are available from Eurocontrol's EAD You'll need an account (free) and Java (which utterly and truly sucks, even though it says it's plain html, it's lying to you, not to mention the applet is really slow)

Yeah I linked to that at the bottom of my answer. Apparently you can change it to html, but you have to log in through the Java version first.

If you look up the ICAO code, e.g. here
http://en.wikipedia.org/wiki/International_Civil_Aviation_Organization_airport_code
then Google for the code + charts, e.g "EGLL charts", you will usually find what you're looking for.
https://www.google.co.uk/search?q=egll+charts

Airport ground charts are always part of the AIP (Airport Information Publication). They are usually publicly available in electronic format, but not always easy to find. Wikipedia's entry for AIP has a
For european airports, you can get them from Eurocontrol (requires registration).
However, you can also usually get these directly from the country's CAA or AIS. Here's the ones you asked for (be aware these links may only work for a short period of time):
Paris CDG.
Amsterdam Schiphol.
Prague.

Just stumbled across this directory, hosted by VATSIM. Doesn't appear to be entirely up to date but does provide a link for charts for Prague, among many others. (Includes links to content that may be behind a paywall though).
http://www.vatsim.net/charts/


text:
How to calculate the Lift Coefficient for the A320?
I'm trying to figure out the lift coefficient of an A320.
What I'm doing is the following: during cruise, the lift is equal to the weight of the aircraft ($mg$) so that I used the cruise table taken from the FCOM to get the speed, the wing area is known ($122.6 m^2$) and I calculated using a tool online the air density at FL290 (29,000 ft).

The result is that the lift coefficient is $0.03029$. Is it right for the assumptions given above?
Next, if I want to relate the $C_L$ with the Angle of Attack, what is the formula and the procedure?

Well.... I doubt that any current airliner will be able to fly with just 0.03 of $C_L$, did you mulitplied the mass by the gravity?

50000kg is the "weight" so already multiplied by g, that's not the mass.

Well, first hint for knowing that the gravity shall be used is that "weight" is measured in Kg instead of N. Second is having a so low $C_L$, having actually lower $C_L$ than usual "C_D$ values. Another warning message is that if 50000 is actual "weight" the mass will be 5000Kg, which means that for an A320 being an airplane of more than 100 passenguers of around 75Kg (in mass) having a total mass of 7500Kg we have a negative mass of the structure of -2500Kg for reaching the the 50000/9,8 mass value. In that sense, I think it is more likely that "weight" is actually "mass" than other option

It's the same as when I say for example: "My weight is 60Kg". This means that my mass is 60/9.8.

My point is that, although it is said "weight" it actually means "mass". Having a $C_L$ of ust 0.03 is unphysical. An airplane of 100 passenguers with a actualy weight of 50kN is not possible ....

The Operating Empty Weight for an A320 is 42600Kg as it's possible to see here: http://en.wikipedia.org/wiki/Airbus_A320_family#Specifications

That's the point, although says Operating Empty Weight is actually Operating Empty Mass

I found a  different number for $C_L$, please check your calculation:
$C_L=\frac{2 m g}{\rho V^2 S}$
substitute:
$
m = 50000 \textrm{ kg}\\
g = 9.81 \textrm{ m}/\textrm{s}^2\\
V = 462 \textrm{ KTAS} = 237.67\textrm{ m/s}\\
S = 122.6 \textrm{ m}^2\\
\rho = 0.475 \textrm{ kg}/\textrm{m}^3
$
This will give:
$C_L = 0.298$

Note that this lift coefficient is for the total aircraft. It includes the lift of the wing, the fuselage lift and also the negative lift of the vertical stabilizer. There is also a small vertical component of thrust that is neglected.

Now that we have an estimate of the lift coefficient, we can estimate the angle of attack. Normally the lift coefficient is assumed to vary linearly with the angle of attack:
$C_L = \frac{\textrm{d}C_L}{\textrm{d}\alpha}\cdot (\alpha-\alpha_0)$
$\alpha_0$ is the zero-lift angle of attack, the angle of attack at which the wing does not generate any lift. For symmetric airfoils, $\alpha_0 = 0$, for chambered airfoils $\alpha_0 < 0$.
I don't know what that angle is for the A320, it will be difficult to obtain. Let's assume for now it is -1.2 degrees.
For infinite long wings in incompressible flow the lift slope $\frac{\textrm{d}C_L}{\textrm{d}\alpha}$ is $2\pi$.
$(\alpha-\alpha_0) = \frac{C_L}{2\pi} =0.0474 \textrm{ rad} \approx 2.72^\circ$
This would result in an angle of attack of $\alpha \approx 1.52^\circ$
Because the wing of the Airbus A320 is not infinitely long but has a span of about 30 meters (excluding the fuselage) we need to correct for that. The reason we need to correct for the finiteness of the wing is that the circulation will cause the local angle of attack of the wing to be lower than the free stream angle of attack. The effective angle of attack $\alpha_{eff} = \alpha - \alpha _i$
The induced angle of attack $\alpha_i$ is given by:
$\alpha_i = \frac{C_L}{\pi AR} = \frac{C_LS}{\pi b^2}$
substitute:
$
S = 122.6 \textrm{ m}^2\\b = 30 \textrm{ m}
$
gives $\alpha_i = 0.0129 \textrm{ rad} \approx 0.74^\circ$
Adding this to our earlier angle of attack results in:
$ 1.52 + 0.74 = 2.26^\circ$
This may not be very accurate as
a) the zero-lift angle of attack may be very different,
b) the lift slope may be flatter due to compressibility effects

The two densities used are slightly different. I think this could explain the 1.6% difference.

@MikeFoxtrot the difference is almost 90%, or 883% depending on which CL you take as denominator...

took the liberty of fixing a comma-decimal dot typo

@Federico thnx!

@DeltaLima Just spotted the decimal points was off so indeed :P The online tool gives a density of 0.47545 as compared to 0.475 that you use.

@DeltaLima $\alpha_0 < 0$ ??? How did you estimate 0 lift angle?

@DeltaLima notice as well that $2\pi$ is the ideal slope without compressibility effects and as well, referred to the "ideal" surface. I think is a good estimation, but if somebody reads this and wants to perform a more detailed estimation  double check the definition of surface area. Is winglet included? Is the are within the fuselage connecting both side of the wing included? That might change the number (is not a simple question...)

@TrebiaProject. I don't understand your first remark, did you expect $\alpha_0 > 0$ ? $2\pi$ is the slope for the infinite wing which has no winglets nor a fuselage. There are so many way

@DeltaLima arg... forget my comment about "\alpha_0" I made a mistake confusing "\alpha_0" and the alpha of minimum drag. For wing efficiency normal actual values are around 3-4. My question as well is how you estimate the value "\alpha_0"

@TrebiaProject. The value $\alpha_0$ was an educated guess. What "normal actual values" are you referring to? Do you mean the $alpha$ during cruise flight is usually in the 3 - 4$^\circ$ range? Note the OEM is 42,600 kg and the MTOM is 78,000 kg, so the aircraft in this question is very light at 50,000kg. That could explain the lowish angle of attack.

@DeltaLima I mean the wing efficiency or wing slope is aroudn 3-4 instead of 6.28. Yep, looks like unloaded airplane.

@TrebiaProject. Ah, I see. Effectively the lift slope is 4.94 in my calculation. Because $\frac{\textrm{d}C_L}{\textrm{d}\alpha_{eff}} = 2\pi$ and $\frac{\textrm{d}\alpha_{eff}}{\textrm{d}\alpha} = \frac{b^2}{b^2\cdot2S}$.

According to A320 FCOM the span (b)=34.6 m not 30m is there any difference in dimensions?

@MikeGolf The difference is approx. 4 meters of fuselage diameter.